1. Field of the Invention
The present invention relates to a reaction cathode characterized by a high thermic emission, which is especially suited for use in vacuum tubes. More particularly, this invention relates to a reaction cathode having a composition containing an emitting, monolayer-forming element which is liberated from the compound by a reaction occurring during cathode operation.
2. Description of the Prior Art
In general, reaction cathodes are known. In these known cathodes, the monolayer (monoatomic surface layer) is stored as a reserve supply in chemically bound form and is steadily liberated from this compound by an appropriately induced reaction throughout the lifetime of the cathode. The rate of the reaction is commensurate with the evaporation rate of the monolayer during the cathode operation, which enables the formation of a monolayer which is sufficient for the desired electron emission current density, as a coating on the cathode in the stationary state.
This type of monolayer-forming reaction cathode is quite equivalent in function to the more common reactionless cathode commonly used for electron emission -- after, of course, some form of activation before being put into operation. To this latter type belong, together with the direct emitting, i.e. without a monolayer cathode (e.g., direct emitting cathodes made from a refractory metal), also certain monolayer-cathodes, viz. alloy cathodes and those storage cathodes in which a monolayer-forming metal is physically stored by capillary action. It is characteristic of the reactionless cathode that the emitting or monolayer-forming substance is present in the cathode in direct emitting or physically stored form and does not have to be continuously liberated from a compound by a supply reaction. In contrast to refractory pure metal cathodes, which permit only low emission current densities, alloy cathodes, for example, allow quite high emission current densities to be obtained; however, the lifetime is insufficient for use in vacuum tubes and is not comparable to those of, e.g., the usual tungsten-tungsten carbide-thorium oxide cathodes.
In FIG. 1 of the attached drawings is a schematic survey of the cathode types mentioned up to now with a further subdivision of the present monolayer-reaction cathode types. Accordingly, the latter type (heavily outlined area in FIG. 1) includes first the group of cathodes utilizing thermal decomposition of an active substance with release of a monolayer-forming element -- called "thermal decomposition cathodes" for short -- and the group of cathodes utilizing a reaction partner in the chemical supply and release reaction -- referred to as "conversion cathodes".
In the thermal decomposition cathodes, the monolayer former is released from an active substance by a, completely or at least essentially, thermally determined decomposition reaction. Examples of this are the familiar tungsten-thorium oxide cathode without reduction means, and the likewise familiar lanthanum hexaboride cathode, neither of which requires any reducing or other reacting components for the liberation of the monolayer-forming thorium or lanthanum for the supply reaction under operating conditions.
In the conversion cathodes the supply and liberation reaction in the operating condition of the cathode proceeds with the participation of a reaction component in the case of an oxide-reducing type cathode, e.g., with a carbon-containing reduction agent.